The invention relates to an SPM sensor for a scanning probe microscope with a cantilever, a holding element at one end of the cantilever and a sensor tip at the other end of the cantilever and with an at least partially cylindrical electron beam induced deposition (EBID or, shorter, EBD) scanning tip protruding from the sensor tip, and also to a method for producing sensors of this type.
Scanning probe microscopes are generally known and are used in practice to scan the surfaces of specimens with fine sensors up to atomic resolution. Scanning probe microscopes (SPM) also include atomic force microscopes (AFM) and scanning tunneling microscopes (STM), which are likewise generally known. Scanning probe microscopes use sensors which comprise a microcantilever, referred to hereafter as the cantilever, which has at its one end a holding element and at the other end a sensor tip, with which the specimen is scanned. In the case of scanning probe microscopy, the scanning tip is brought up closer to the surface under investigation until the probe interacts with the surface under investigation. On account of the atomic forces acting between the scanning tip and the surface, the cantilever of the sensor is deflected. The detection of this flexural distortion makes it possible to measure the topography and other properties of the surface of the specimen.
The resolution is determined here by the form of the probe tip that interacts with the surface of the specimen during the measurement. The result of the measurement with the scanning probe microscope is always a convolution of the form of the SPM sensor and the actual form of the surface of the specimen. As a result of the manufacturing process, the sensor tip is of a pyramidal, conical or cylindrical form and generally has a height of a few (typically 5-20) micrometers and a typical aperture angle of 20 to 70 degrees. The maximum resolving power of sensors of this type when measuring the topography of a surface of a specimen lies in the range of below one nanometer to several nanometers. Owing to the geometry of the SPM sensors, it is therefore not possible, in particular in the case of specimens with a pronounced topography and structures with a high aspect ratio, to reproduce the actual form of the surface. For the detection of fine structures and indentations, the resolving power of a sensor can be significantly improved by reducing the size of the aperture angle of the sensor tip. It is known to reduce the size of the effective aperture angle of the sensor tip by depositing an additional scanning tip in needle form, referred to hereafter as the EBD scanning tip, on the lateral surface of the sensor tip, and in this way improve the resolving power. This specially formed EBD scanning tip is produced by bombarding the probe tip with electrons in a vacuum and the deposition of material thereby induced (contamination, gas atoms or molecules, usually carbon or organic carbon compounds) at the location irradiated by the electrons. This effect is referred to as electron beam induced deposition (EBID or, shorter, EBD) and has already been the subject of many publications.
When scanning the surface of a specimen with the probe or with the EBD scanning tip, great forces act on the EBD scanning tip. In order to withstand the forces, and obtain a long service life of the grown-on tip, the EBD scanning tip in needle form must be durably connected to the tip of the SPM sensor. DE 198 25 404 A1 discloses the production of an EBD scanning tip on an SPM sensor, grown on by electron beam induced deposition, and methods for fastening/anchoring it. The EBD scanning tip is stabilized by additional supporting structures, likewise produced by electron beam induced material deposition, which are deposited on the surface of the sensor tip and laterally support the actual measuring tip. The practical use of sensors produced in this way shows that this fastening method does not adequately stabilize the grown-on EBD scanning tips and that the EBD scanning tips often become detached from the surface of the sensor tip during measurement.
EP 1 278 055 A1 discloses an SPM sensor which has at its sensor tip a scanning tip formed as a “carbon nanotube”. The nanotube is produced in a process of its own and fastened in a hole in the substrate of the sensor tip. It has a cylindrical form with a typical diameter of 1-10 nm. The fastening may take place by means of an electron beam induced coating or by electron beam induced or current flow induced thermal fusion on the substrate.
WO 00/19494 discloses a method for producing “nano-holes” in substrates of SPM sensors for the fastening of carbon nanotubes which have a diameter of 1-10 nm. In this case, a hole that is larger than the diameter of the nanotube is first created and then the diameter of the hole is adapted to the diameter of the nanotube by coating its wall.
The present invention is therefore based on the object of proposing an SPM sensor which has a greater stability and resistance of the deposited EBD scanning tips, and consequently a longer service life.